This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-200174, filed Jun. 29, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a metallic closed vessel, or a so-called canister, in which a radioactive substance that involves heat release is sealed, a seal-welding method for the closed vessel, and an exhaust system used for the seal-welding method.
2. Description of the Related Art
Highly radioactive substances represented by spent fuels from nuclear reactors are destructured and reprocessed in order to recover plutonium or some other useful substances that can be used again as fuels. These spent fuels are contained in closed places before they are reprocessed. Known containing methods for these highly radioactive substances include a wet method that uses storage pools and the like and a dry method that uses casks and the like.
The dry method is a containing method in which air is used in place of water for natural cooling. Since the running costs of the dry method are lower than those of the wet method, the dry method has started to attract attention and be developed. Known casks that are applicable to the dry method include metallic casks and concrete casks based on a concrete structure for shielding the spent fuel. Each of these casks is provided with a tubular vessel body that is closed at both ends, top and bottom. The spent fuel is sealed in a tubular metallic closed vessel or a so-called canister, moreover, the canister is put into the vessel body of the cask. Thus, radioactive substance can be contained in a shielded state.
Usually, the canister comprises a tubular vessel body closed at the bottom and a lid that closes a top opening of the vessel body. A basket is located in the vessel body, and a plurality of spent fuel assemblies are sealed in the vessel body in a manner such that they are supported by the basket. Normally, the spent fuel assemblies are sealed into the canister in the following processes.
First, the open-topped vessel body of the canister is immersed in cooling water and filled with the water. In this state, the basket and the spent fuel assemblies are contained in the vessel body. Thus, the spent fuel assemblies are temporarily shielded with the cooling water to prevent leakage of radiation.
Subsequently, a primary lid is dropped onto the top opening of the vessel body to close it, and a suitable quantity of water is discharged. Thereafter, the primary lid is welded to the vessel body to seal the top opening of the vessel body. After the water is completely discharged from the vessel body through a drainage hole in the primary lid, the drainage hole is sealed. Further, a secondary lid is lapped onto the primary lid and welded to the vessel body. Thus, the resulting canister has the spent fuel assemblies well sealed therein.
In the sealing process for the canister described above, the vessel body is filled with the cooling water as the primary lid is welded to it, in order to intercept radiation from the spent fuel assemblies. However, the welding operation takes so much time that the cooling water in the vessel body is heated by the spent fuel assemblies and evaporated gradually. The resulting steam fills the vessel body and flows out of it through the gap between the inner surface of the vessel body and the primary lid.
Normally, a welding operation is performed in a manner such that molten deposited metal naturally drops by the gravity, thereby forming penetration beads. As this is done, however, steam gets into the gap between the inner surface of the vessel body and the primary lid, as a welding portion, so that weld defects such as voids are inevitably formed in the welding portion. These weld defects lower the strength of the welding portion, and a radioactive substance leaks from the defective portions. Thus, it is hard to maintain the integrity or radioactive substance sealing performance of the canister.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a metallic closed vessel free from weld defects and high in sealability, a seal-welding method for the closed vessel, and an exhaust system used for the seal-welding method.
In order to achieve the above object, a closed vessel according to an aspect of the invention comprises: a substantially tubular vessel body closed at the bottom, having a top opening, and configured to contain radioactive substance; and a lid set in the top opening of the vessel body and welded to the inner peripheral surface of the vessel body.
The lid has an outer peripheral portion adjacently opposed to the inner peripheral surface of the vessel body, the outer peripheral portion including a welding portion welded to the inner peripheral surface of the vessel body and a space portion located on the bottom side of the vessel body with respect to the welding portion. The space portion is configured to be filled with a shield gas or to allow the flow of the shield gas therein so as to shield the welding portion from the interior of the vessel body, as the welding portion is welded.
A closed vessel according to another aspect of the invention comprises: a substantially tubular vessel body closed at the bottom, having a top opening, and configured to contain radioactive substance; a shielding plate set in the top opening of the vessel body and closing the top opening; a seal member for sealing a gap between the inner peripheral surface of the vessel body and the shielding plate; and a lid set in the top opening of the vessel body so as to be lapped on the shielding plate and having a peripheral edge portion welded to the inner peripheral surface of the vessel body. The lid has an outer peripheral portion adjacently opposed to the inner peripheral surface of the vessel body, the outer peripheral portion including a welding portion welded to the inner peripheral surface of the vessel body and a space portion located on the bottom side of the vessel body with respect to the welding portion. The space portion is configured to be filled with a shield gas or to allow the flow of the shield gas therein so as to shield the welding portion from the interior of the vessel body, as the welding portion is welded
According to the closed vessel for a radioactive substance constructed in this manner, steam can be prevented from getting into the welding portion by filling into or running the shield gas through the space portion of the lid as the lid means is welded. Thus, the lid can be securely welded without involving any weld defects that are attributable to steam.
Since the gap between the shielding plate and the vessel body is sealed, moreover, steam can be more securely prevented from getting into the welding portion through the gap as the lid means is welded. In consequence, the lid means can be securely welded without involving any weld defects that are attributable to steam. Thus, the resulting closed vessel provides improved integrity and high radiation shielding properties.
A seal-welding method for a closed vessel configured to contain radioactive substance according to still another aspect of the invention comprises: filling water into a substantially tubular vessel body closed at the bottom and having a top opening; placing a radioactive substance in the vessel body and immersing the substance in the water; setting a lid in the top opening of the vessel body to close the top opening; evacuating the vessel body through a discharge hole formed in the lid and discharging steam generated in the vessel body to the outside, while charging air into the vessel body through the discharge hole; and welding a peripheral edge portion of the lid to the vessel body, thereby sealing the top opening of the vessel body, while discharging the steam to the outside.
A seal-welding method for a closed vessel according to a further aspect of the invention comprises: filling water into a substantially tubular vessel body closed at the bottom and having a top opening; placing a radioactive substance in the vessel body and immersing the substance in the water; setting a shielding plate in the upper end portion of the vessel body to close the top opening, and sealing a gap between the inner peripheral surface of the vessel body and the shielding plate by means of a seal member; setting a lid in the top opening of the vessel body to be lapped on the shielding plate, thereby closing the top opening; evacuating the vessel body through a discharge hole formed in the lid and the shielding plate and discharging steam generated in the vessel body to the outside, while charging air into the vessel body through the discharge hole; and welding the peripheral edge portion of the lid means to the vessel body, thereby sealing the top opening of the vessel body, while discharging the steam to the outside.
According to the seal-welding method for a closed vessel of the invention, moreover, the lid has an outer peripheral portion adjacently opposed to the inner peripheral surface of the vessel body, the outer peripheral portion including a welding portion welded to the inner peripheral surface of the vessel body and a space portion located on the bottom side of the vessel body with respect to the welding portion, and a shield gas is filled into or run through the space portion, thereby preventing the steam from getting into the welding portion, as the lid means is welded.
According to the seal-welding method for a closed vessel described above, the vessel body is evacuated to discharge steam as the lid is welded, whereby the steam can be prevented from getting into the welding portion. Thus, the lid can be securely welded without involving any weld defects.
Further, the steam can be more securely prevented from getting into the welding portion in a manner such that the shield gas is filled into or run through the space portion of the lid as the lid is welded. The resulting closed vessel enjoys high closeness and satisfactory radioactive substance sealing properties without involving any weld defects.
Furthermore, an exhaust system according to the invention comprises: a charging pipe configured to be passed through the discharge hole and having a charging port opening into the vessel body and a suction port opening to the outside of the vessel body; an exhaust pipe located in the charging pipe to form a double-pipe structure and having an exhaust port opening into the vessel body and an extending portion extending to the outside of the vessel body; and a suction device connected to the extending portion of the exhaust pipe and configured to evacuate the vessel body through the exhaust pipe and charge the open air into the vessel body through the charging pipe.
According to the exhaust system constructed in this manner, the vessel body can be simultaneously exhausted and charged by using the one discharge hole. More specifically, the air containing steam in the vessel body is discharged through the exhaust port by means of the suction device, and in concert with this, air is charged into the vessel body through the charging pipe, whereby the internal pressure of the vessel body is regulated. Thus, the steam that is generated in the vessel body can be discharged from the vessel body, so that a large quantity of steam can be prevented from getting into the welding portion. Even though radiation from the radioactive substance is intercepted by means of the water during the welding operation, therefore, satisfactory circumstances can be enjoyed without involving any voids in the welding portion, and improvement of the welding accuracy can be expected.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.